The presence of residual particulates and hydrogen cyanide in various gas streams, especially synthesis gas streams, complicates removal of additional impurities, e.g., removal of H.sub.2 S and/or CO.sub.2, and poses problems insofar as product quality is concerned. Additionally, in some synthesis gas streams, the presence of reduced iron in the flyash particulates results in the formation of minor amounts of iron cyanide complexes in scrubbing solutions employed in scrubbing the synthesis gas. These iron cyanide complexes present disposal problems, since cyanide concentrations of any type in waste disposal streams are severly restricted by environmental authorities.
If the gas stream is scrubbed with sufficient water or aqueous scrubbing solution, e.g., as described by Diaz in U.S. Pat. No. 4,497,784, entitled "Solution Removal of HCN from Gaseous Streams, with Hydrolysis of Thiocyanate Formed" issued Feb. 5, 1985; U.S. Pat. No. 4,505,881, entitled "Ammonium Polysulfide Removal of HCN from Gaseous Streams, with Subsequent Production of NH.sub.3, H.sub.2 S, and CO.sub.2 ", issued Mar. 19, 1985; U.S. Pat. No. 4,508,693, entitled "Solution Removal of HCN from Gaseous Streams, with pH Adjustment of Reacted Solution and Hydrolysis of Thiocyanate Formed", issued Apr. 2, 1985; and U.S. Ser. No. 685,708, filed Dec. 24, 1984 by G. R. Moore, all incorporated herein by reference, the great bulk of the HCN and the remaining particulates will be removed. In the polysulfide treatment, high temperature or acid hydrolysis described therein, the iron cyanide complex or complexes may also be hydrolyzed. If, however, a limited volume of water or high scrubbing temperatures (or both) are employed, the bulk of the HCN will not be removed from the synthesis gas during scrubbing. On the other hand, if sufficiently low scrubbing temperatures are employed, at least a portion, perhaps the great bulk, if sufficient water is used, will be absorbed. Regardless of whether or not the bulk of the HCN is absorbed, the iron cyanide complex or complexes are produced in the scrubbing mixture. The invention addresses this problem.